Recent fine semiconductor devices require the minimization of a pattern width (line width), a pitch between patterns and the like of a circuit pattern to be formed on a wafer and or a resist pattern for forming the circuit pattern and the like. This minimization requirement can be dealt with by shortening the wavelength of ultraviolet light to be used as exposure light to resist. As miniaturization of semiconductor devices progresses more, the wavelength of ultraviolet light to be used as exposure light is shortened to, for example, a wavelength of 365 nm for semiconductor devices under a 350 nm design rule, a wavelength of 248 nm for semiconductor devices under a 250 nm and 180 nm design rule, and a wavelength of 193 nm for semiconductor devices under a 130 nm and 100 nm design rule, ultraviolet light having a wavelength of 157 nm being now in use.
It is known that a resolution relative to a wavelength is generally expressed by the Rayleigh's equation w=k1×(λ/NA) where w is a minimum width pattern to be resolved, NA is a numerical aperture of a lens in a projection optical system, λ is a wavelength of exposure light and k1 is a process constant. The process constant is determined mainly by the performance of resist, selection of ultra resolution techniques and the like. It is known that k1 can be selected to be about 0.35 if optimum resist and ultra resolution techniques are used. According to the ultra resolution techniques, ± first order refraction light of light transmitted through a mask and refracted by a mask light shielding pattern is selectively used to obtain a pattern smaller than the wavelength.
It can be known from the Rayleigh's equation that the minimum pattern width capable of being dealt with if a wavelength of, for example, 157 nm is used, is w=61 nm by using a lens with NA=0.9. Namely, if a pattern width narrower than 61 nm is to be obtained, it is necessary to use ultraviolet light having a wavelength shorter than 157 nm.
For this reason, studies have been made recently to use light having a wavelength of 13.5 nm called extreme ultraviolet (EUV; Extreme Ultra Violet) light as ultraviolet light having a wavelength shorter than 157 nm. Since there is light transmission material such as CaF2 (calcium fluoride) and SiO2 (silicon dioxide) for ultraviolet light having a wavelength of 157 nm or longer, it is possible to form a mask and an optical system capable of transmitting the ultraviolet light. However, for the extreme ultraviolet light having a wavelength of 13.5 nm, material capable of transmitting the extreme ultraviolet light at a desired thickness does not exist. Therefore, if the extreme ultraviolet light having a wavelength of 13.5 nm is used, a mask and an optical system of a light transmission type cannot be used, but rather a mask and an optical system of a light reflection type is required.
If a mask and an optical system of the light reflection type are used, light reflected from a mask surface is required to be guided to a projection optical system without being interfered with by light incident upon the mask. It is therefore essential that light incident upon the mask is required to be oblique at an angle φ relative to the normal to the mask surface. This angle is determined from the numerical aperture NA of a lens in a projection optical system, a mask multiplication m and a size σ of an illumination light source. Specifically, in an exposure apparatus with NA=0.3 and σ=0.8, light is incident upon a mask, having a solid angle of 3.44±2.75 degrees. If a mask having a reduction factor of 4 relative to a wafer is used and an exposure apparatus has NA=0.25 and σ=0.7, light is incident upon the mask, having a solid angle of 3.58±2.51 degrees.
As a reflection type mask for use with oblique incidence light, a mask blank is known which reflects extreme ultraviolet light and has an absorption film covering the mask blank with a predetermined pattern and absorbing extreme ultraviolet light and a buffer film interposed between the mask blank and absorption film. The mask blank has the structure that an Si (silicon) layer and an Mo (molybdenum) layer are alternately stacked, and the repetition number of stacks is generally 40 layers. Since the absorption film for extreme ultraviolet light covers the mask blank with a predetermined pattern, incidence light is selectively reflected in accordance with a circuit pattern to be formed, a resist pattern or the like. The buffer film is formed, as an etching stopper when the absorption film is formed, or in order to avoid damages to be caused when defects are removed after the absorption mask is formed.
As described above, a conventional mask blank has generally 40 layers as the repetition number of stacks of the Si layer and Mo layer. A reflectance of Si is 0.9993–0.00182645i and a reflectance of Mo is 0.9211–0.00643543i, where i is an imaginary unit. It is known that a proper ratio Γ of a Mo layer thickness to a total thickness of the Si layer and Mo layer is Mo layer thickness'(Si layer thickness+Mo layer thickness)=0.4. Therefore, in a conventional mask blank, if the wavelength λ of extreme ultraviolet light to be used for exposure is 13.5 nm, the total thickness of the Si layer and Mo layer is (λ/2)/(0.9993×0.6+0.9211×0.4)=6.973 nm, a thickness of the Si layer is 6.9730×0.6=4.184, and a thickness of an Mo layer is 6.9730×0.4=2.789 nm. FIG. 1 shows a reflectance of the mask blank having 40 layers of the stack of the Si layer and Mo layer described above. In the example shown in FIG. 1, the reflectance is at an incidence angle of 4.84 degrees. The incidence angle is defined as an angle relative to the normal to the surface of the mask blank.
The alternately stacked Si layer and Mo layer structure is used not only for a mask blank of the reflection type but also for a reflection mirror constituting a reflection type optical system in quite a similar manner. Namely, the reflection mirror for extreme ultraviolet light has generally 40 layers as the repetition number of stacks of the Si layer and Mo layer, and the reflectance shown in FIG. 1 is obtained by properly setting the thicknesses of the Si layer and Mo layer when the wavelength of extreme ultraviolet light is 13.5 nm.
Extreme ultraviolet light generally propagates via a plurality of reflection surfaces from a light source of an exposure apparatus to resist coated on a wafer, for example, six mirror reflection surfaces of an illumination optical system, six mirror reflection surfaces of a projection optical system and one reflection surface of a mask, thirteen surfaces in total. Extreme ultraviolet light emitted from the light source is attenuated upon reflection at a reflection surface. If this attenuation is large, sufficient energy cannot reach the resist coated on the wafer and there is a possibility that pattern formation and the like cannot be performed properly.
If extreme ultraviolet light propagates via a plurality of reflection surfaces, the energy reaching the resist coated on a wafer can be estimated from a reflectance at each of the plurality of reflection surfaces and a light source intensity. A reflectance R via a plurality of reflection planes is given by the following equation (1) if the light propagates via thirteen reflection surfaces in total. RTE is a reflectance of a TE wave per one reflection surface and RTM is a reflectance of a TM wave per one reflection surface.R={(RTE+RTM)/2}13  (1)
A reflectance R of thirteen surfaces in total was obtained by using the equation (1) when the mask blank and reflection mirrors having the reflectance shown in FIG. 1 are used. The reflectance R is as shown in FIG. 4. It can be seen from the example shown in FIG. 4 that the center of the half width of a spectrum of the reflectance R is not coincident with 13.5 nm which is the center wavelength of exposure light of extreme ultraviolet light. Namely, even if the center of FWHM (Full Width at Half Maximum) of a reflectance per one reflection surface is coincident with the center wavelength of exposure light (refer to FIG. 1), the center of FWHM of the reflectance R via thirteen reflection surfaces in total is not necessarily coincident with the center wavelength of exposure light and the wavelength dependency may deviate from the center wavelength of exposure light. This results from the fact that the peak wavelength for the reflectance per one reflection surface is not coincident with 13.5 nm which is the center wavelength of exposure light of extreme ultraviolet light. As above, if the wavelength dependency of the reflectance via a plurality of reflection surfaces deviates from the center wavelength of exposure light of extreme ultraviolet light, attenuation at the center wavelength of exposure light, i.e., attenuation of a light source intensity of the light source, becomes large. Therefore, at an exposure light wavelength suitable for resist coated on a wafer, sufficient energy will not reach the wafer and the probability that pattern formation and the like cannot be performed properly becomes very high.
It is therefore an object of the present invention to provide a reflector for exposure light which can retain a sufficient energy reaching a subject to be exposed, by making the wavelength dependency of a reflectance via a plurality of reflection surfaces be coincident with the center wavelength of exposure light such as extreme ultraviolet light.
In a lithography process for manufacturing a semiconductor device, a number (a variety) of exposure masks are used in some cases. Further, if there are a plurality of exposure apparatuses and manufacture is executed at a plurality of factories, a plurality of exposure masks are often used even for the same product and even in the same process. In such cases, it is fairly conceivable that thicknesses of films and the like constituting each of a plurality of exposure masks have manufacturing variations.
The manufacturing variations of this type, i.e., a thickness variation of films and the like constituting each exposure mask, causes a deviation of the center of FWHM of the reflectance relative to extreme ultraviolet light, which may result in a reduction in arrival energy at an exposure light wavelength suitable for resist coated on a wafer. It is therefore desired to remove the variation as much as possible. However, for example, when the productivity of mask blanks is considered, it is not realistic to limit the film thickness and the like too severely.
It is therefore an object of the present invention to provide a reflector for exposure light, its manufacture method, a mask, an exposure apparatus and a semiconductor device manufacture method, which can retain a sufficient energy reaching a subject to be exposed, by making the wavelength dependency of a reflectance via a plurality of reflection surfaces be coincident with the center wavelength of exposure light such as extreme ultraviolet light.